Higher fidelity in AM radio reception in tube sets, and a really good one using cathode followers as buffers of the detector, and better audio bandwidth in tube and solid state, including AM/FM stereo tuners/ receivers. Adding a tube "preamp" stage to a solid state receiver/amplifier to have "tube sound" And a tube output stage in a solid state radio. Cathode followers in an AM/FM stereo solid state tuner as output line buffers , and in a Sony receiver.
Wide band AM IF filter for better AM station audio reception.

Using "Grid stoppers" to supress parasitic supersonic oscillations in audio amps

Sometimes an audio amp will present strange behavior. Strange hum behavior is one, interference that seems sensitive to volume control settings or closeness to external objects in an AM radio can sometimes be cured by using a grid stopper. Parasitic supersonic oscillations tend to occur with high gain tubes. Tubes like a 12AX7 or 12AV6. The grid stopper resistor in combination with the Miller capacitance of the tube as seen by the grid works as a low pass filter to prevent the parasitic supersonic oscillation. Guitar amps using 12AX7s often use a 68K resistor connected physically as close as possible (using a short lead) to the grid pin of the tube. This resistor is in series with (the coupling cap and resistor to ground) circuit. An example is seen in the below diagram. The MIller capacitance of a 12AX7 is around 150pF, and this forms a low pass RC filter at around 100KHz.

Sometimes a high gain output tube needs a grid stopper. An example I had was that a 60HL5 needed a "grid stopper" resistor of around 4.7K to avoid oscillations at supersonic and RF frequencies. This tube has a high gm. The resistor and the stray capacitances in the tube form a low pass filter that kills gain at frequencies above audio, thus stopping the oscillations. These oscillations can cause audio distortion and can also trash some AM band frequencies. I used a surface mount resistor to bridge across a cut I made in the circuit board trace feeding the 60HL5's control grid. Makes for a neat install.

Using feedback in AA5 tube radio audio amps for better fidelity

Fig 1:

Fig 3:

More bandwidth on the IF for higher AM reception fidelity, down below the feedback section.

If you have a particularly good sounding AA5 radio with a nice speaker and cabinet (or you could replace the speaker with one intended for car audio), it is possible to add some negative feedback for better fidelity. Some reduction in hum can also be had with negative feedback. There's usually plenty of audio gain avaliable in an AA5 tube radio to allow some use of feedback, as feedback will reduce gain. At first I tried some feedback into the cathode of the 12AV6, but this cathode also serves the diode detector circuit. Oh, it worked, but the feedback ends up being applied to two paths, which, depending on the volume control setting, would tend to cancel out. One solution to this problem is to use a twin triode tube like a 12SL7 or 12AX7, using one section as the triode audio stage with the feedback going into the cathode. Place an additional 56K resistor from the plate to B+ to make the plate load 47K. And use a 2K resistor on the cathode, this cathode resistor bypassed to the voice coil output transformer secondary. Doing this gives us about 2 dB of additional gain, as the plate impedance is lowered. Thus the output tube grid resistor won't load it as much. I used a surface mount resistor on the bottom of the AA5's circuit board. The other triode section is wired as a detector diode, grid to the IF transformer, plate and cathode to ground. See Fig 2 below.

Another method without this drawback is possible. It involves adding a third winding to the audio output transformer. About ten turns around one of the outer legs of the iron core laminations will do. See Fig 1. But as this winding will be carrying the audio input signal, it will need to be shielded from the other windings of the transformer. Use some copper foil tape between this new winding and the other windings of the transformer. And also use some foil tape to shield the windings from the outside world. Ground these shields and the transformer laminations. Be sure not to create a shorted turn around the leg of the iron core. And shield the two leads of this new winding from the transformer back to the 12AV6 circuit. Or if there's enough room in the transformer, you could use very thin coax cable for the new windings. Ground only one end of the shield of the coax. In Fig 3, I didn't need the foil tape. I grounded the transformer laminations and also grounded the voice coil. The voice coil winding (being the outer winding over the primary winding) being grounded acts to shield the new feedback winding from the primary winding. This feedback winding subtracts a little bit of the output signal from the input signal without affecting the detector circuit. That then creates the negative feedback loop. If phased properly, you should be able to short the feedback winding and hear the audio level *increase* a couple of dB, and have less fidelity.

The capacitor usually connected to the plate of the output tube can be reduced to a few hundred pF's, or removed completely. The feedback loop will now keep the radio from sounding shrill.

Which way is the audio transformer phased? We need a negative feedback loop. Phased the wrong way we will get an oscillator instead of an amplifier! If that happens, reverse the leads on the audio output transformer's new winding at the 12AV6 (Fig 1), or the speaker driver winding if you used feedback into the first audio triode's cathode (Fig 2).

For more bass, install a 0.1uF cap from the plate of the triode driver stage to the output tube's control grid. But don't install a bigger cap on the grid of the triode driver to the volume control, as it could back bias the diode detector more, see below.

Fig 2:

Modify the detector circuit for better modulation acceptance

Modulation acceptance is the amplitude modulation percentage (aka modulation index) a receiver's detector can handle without distortion. An ordinary diode detector can handle upward modulation well but detector circuits with capacitive loads have limited ability to faithfully reproduce downward modulation. At some point they clip the audio waveform before getting to zero. Typically an AA5 will start to clip modulation that drops below about 20%. FCC rules say the minimum is about 5%. This level is when the music is at the loudest. The RF carrier varies between 5% and 195% of the "dead air" RF level when the music is loud. This is known as "95% modulation". AA5s tend to clip on modulation above the high 80's.

Assuming that the signal level at the detector is high enough to avoid the "knee" (non-linear region just above zero volts, which if you are using a 6AL5 can be reduced by running its heater at 4V) of the detector diode and only the use of RC filter caps to remove the IF frequency from the detected audio, you can get around -50dB distortion products in the audio out. This RC filter is the 100pF caps and the 47K resistor. If you then connect an AVC filter network to the audio output, the 0.05uF cap will take longer to discharge than the period of time of the typical audio waveform. Even though the AVC resistor (usually 2.2 or 3.3 megs) and the volume control (usually 500K) form a voltage divider for the voltage stored in the AVC cap, about 20% of the AVC voltage shows up at the detector diode. This causes the diode to be back biased, requiring extra voltage from the AM signal to conduct. This has the effect of clipping off the bottom excursion of the AM signal's carrier's modulation.

An easy way to improve this situation is to make use of the second diode in the 12AV6 or 12SQ7 tube. Disconnect the AVC resistor from the volume control or the IF transformer and then connect it to the anode of the extra diode. Add a new 470K resistor to that diode's anode and the other end to the 12AV6 cathode (usually ground). And finally connect a 30pF cap between the plate of the IF amp tube and the newly used 12AV6 diode anode. See diagram below. This gives us AVC action without messing up the detected audio, as they are now separate circuits. This requires two extra parts in an AA5, something manufacturers would avoid in a mass market consumer product. This mod may not make much difference in a small table set, but it should greatly improve a large console set or the AM section of a stereo system.

The audio coupling cap from the volume control wiper to the grid of the 12AV6 can also add to the problem, but the voltage divider effect between the wiper resistance to ground and the 10 meg resistor pushes it down to a per-cent or so. And this cap, being smaller, would discharge faster. A cathode follower could be used as a buffer to avoid even this, but the cost would preclude its use except in a very high end set.
Another thing you should address is the small RF filter caps on the audio output of the detector. Have them too big and you can get tangent distortion because they take too long to discharge to allow a full amplitude 5KHz sine wave thru without getting tangent clip. Change the (usually) 100pf caps to around 33 to 47pF.

Reduced heater voltage on detector diodes like the 6H6, 6AL5 or 5896 can improve detector performance on weak signals. Less "contact potential" for the signal to overcome. This should increase the fidelity of AM detection. The 5896 below is a dual diode version.

This reduced heater trick seems to also work well on 12SQ7's and 12AV6's. Only thing is to keep in mind that if you reduce the contact potential for the diodes, it also drops for the triode. But it seems that running the heater at 10V vs 12.6 seems to be a sweet spot. Parallel a 330 ohm half watt resistor with the heater in a series string set. If it's a 6SQ7 in a 300ma heater string, use 160 ohms if you have it, or use 150 ohms. For a 18FY6, a 510 ohm resistor would be about right. It shouldn't hurt the tube, as the current demand on the triode in an AA5 is quite low. The below graph (original source before edits: http://www2.famille.ne.jp/~teddy/datalib/heater.htm) shows the impact of reduced heater voltage on the 12AV6 triode. At 10V heater voltage the plate current curves are slightly shifted to lower plate currents vs plate voltage, but not adversely so. Around 9V things start to fall apart.

You can use two detector diodes in a solid state AM radio as well, for similar reasons. Namely, keeping the AVC filter cap from back biasing the detector diode. I kept the silicon diode (D3) (600mV diode drop) that came with the radio for the AVC circuit (R29 and a new 0.01uF cap to ground), and used a second diode that has a much lower diode drop, around 200mV for the audio signal detector function. I split off the audio signal path (R30, C38) off the now AVC diode and put it on the new diode. Some Schottky diodes can be used, like small-area Schottky diodes (but probably not the rectifier diodes from switching power supplies), but you should try a selection of such diodes you got out of your junk box. As some will work well as AM detectors and others won't. Hard to tell them apart with only your DVM set to diode mode (don't touch the leads with your fingers when you try them in the radio circuit, use needlenose pliers). A sharper diode knee will make for better reception of weak stations, if you add bias to overcome diode drop voltage. This is done with some forward bias current to improve weak signal detection, as this bias will help overcome the diode drop loss of signal present at the diode. A diode with a higher drop but sharper knee would be the better choice. Too many variables to calculate the resistor value to use (what current on the diode knee will make it detect best, what's your supply voltage, what's the voltage of the diode drop voltage, what's the resistive load on the diode circuit such as the volume control, and so on), so trial and error would be quickest.

Also try a small signal silicon transistor with its collector and base tied together. Think doing it like this makes the collector turn on once the base is biased on, making a sharper knee. And collector saturation doesn't seem to be an issue. The emitter on such an NPN transistor would be the cathode, and the base tied to the collector is the anode Here I used a 2SC2785 salvaged from a VCR or such. If the transistor is a PNP, its emitter would be the anode, and its base tied to its collector the cathode. Try various transistors. I found that 680K seems to hit the sweet spot in this radio, but it's not a critical adjustment on a trimpot (which you can use to quickly find the value you'd want in a fixed resistor). I had thought that using a transistor this way was the mark of a sloppy company (did they run out of diodes?), but I got the best weak station detection performance with this. Oh, check strong stations too, which worked well too.

Some simulations of a diode, a base-collector strapped transistor, and such a transistor with a bias current:

I'm not sure how much, if any, knee can be below 0V here. Conduction on the other side of 0V would probably mess up the detection. IIRC, the FCC requires that 10% be the minimium amplitude of an AM station's carrier, which should permit some knee curvature of stations that aren't too weak (else signal to noise would make them unlistenable anyway).

Another angle in getting better reception of weak AM radio stations. This one is to restrict RFI riding on the powerline from getting into the radio. Took an EMI filter network from an old computer monitor (or a PC power supply), like this below (part values are not at all critical):
and placed it between the radio's power cord and the powerline input of the radio circuits. I cut out the section of circuit board from the power supply or monitor that has these parts and used it directly here. Use everything from the the old power supply's powerline input to its bridge rectifier. I'd also leave its ground not connected. We want to keep RFI from riding in via any ground or power wire. Added an extra coil too. Position the filter as far away from the AM loop or ferrite rod antenna as practical, to avoid magnetic coupling into the antenna.

You can tell if this RFI filter mod is a reasonable thing to do by this test: While listening to a weak station with powerline RFI buzzes, disconnect the power plug and quickly listen to see if the buzz goes away before the station and the radio dies. We're talking about 1/4 second, before the main filter caps in the radio's power supply looses charge. You can repeat this test after installing this filter and see if any buzz left over still stays on the station after pulling the plug. Idea is that you effectively pulled the plug on the path the RFI was taking. That your plug in radio acts like a portable on batteries. This mod almost makes it seem like the early 70's on the AM band, before switching power supplies existed.

Here I found room for the double coil on the board of this radio:
Though it's only one coil and cap here, the coil has enough inductance to reduce RFI to a very low level. Note that the coil is angled to be perpendicular to the antenna coil, to avoid coupling.

Even better AM detector using cathode followers and vacuum tube diodes

For a "hi-fi" AM detector that can handle a wide range of signal levels, and where you are willing to use extra tubes, a detector that is double buffered by cathode followers works well. Distortion down about 50dB and about 40dB for really weak signals (other detectors destroy weak signals). The first cathode follower buffers the radio station carrier from the last IF transformer. This allows heavy loading by the detector. Then there is a pair of detector vacuum tube diodes, one for the AVC, and the other for audio extraction. Then the recovered audio is directly coupled to a second cathode follower. This keeps to an absolute minimum the capacitive loading of the detector circuit. Especially no coupling capacitor to back bias the detector diode. With C3, R4, C2 and R5 carefully selected for a good AC/DC ratio, this circuit can handle input signal levels from 20Vp-p to as low as 1mVp-p, with distortion levels of -50dB to -40dB respectively. The second cathode follower needs a negative supply to keep it linear on larger audio signals. I obtained -8V by rectifying the 6.3VAC heater supply in the Heathkit tuner I modified with this circuit. Another way is to bias the audio detector circuit's ground up to around +25V. This gives plenty of headroom for the 2nd cathode follower to handle large signals. Once the simulations looked good I built hardware, and it sounds very good. The IF transformer secondary feeding the first cathode follower has a 100K resistor added across it to widen its passband now that the detector load isn't on it (not shown in the below diagram).

At the expense of sensitivity, the audio bandwidth response of an AA5's RF and IF can be improved. This involves lowering the "Q" of the tuned circuits in the antenna and IF strip. Install a 100K resistor across the secondary of the first IF transformer. This should lower the "Q" to a value that will yield about 15 to 20KHz bandwidth. The secondary of the second IF transformer will be loaded with an extra 100K load on the detector output. This should improve distortion as well as lower the transformer's "Q".

This distortion improvement is due to a better AC/DC load ratio on the detector. The DC load is the resistance directly connected to the detector. The AC load is the DC load with the addition of the resistance on the other side of the coupling cap feeding the grid of the triode.

To avoid "negative peak clipping" you want the AC/DC load ratio to be as close to 1.0 as possible. The further the ratio is from 1.0, the lower the modulation level will be where "negative peak clipping" sets in. The actual negative modulation percentage where negative peak clipping starts is determined by the source impedance of the IF stage driving the detector, and the diode characteristics, as well as the AC/DC load ratio. For comparison a typical "AA5" radio has a DC detector load of 547K and a worst case AC detector load of 433398 Ohms with the Volume control at maximum, for a AC/DC load ratio of 0.792. With the change of the detector load from 500K to 83K (100K in parallel with the 500K pot), the DC load becomes 83K, and the AC load becomes 81K. The resulting AC/DC load ratio of 0.975 is a considerable improvement over the AC/DC load ratio of 0.792 as originally designed. (John Byrns, with edits)

The signal level of each circuit will drop about 6 dB however. You might be able to get some gain back by connecting a small 15pf or so cap from the hot side of the primary to the hot side of the secondary of the IF transformers. This depends on the phasing of the magnetic coupling inside the transformer, however. The antenna circuit's "Q" can also be lowered. In this circuit, one would like an approximately constant bandwidth over the range of the AM MW band. A small resistance in series with the antenna coil before it connects to the tuning capacitor and converter tube will do this. Install a 27 ohm resistor here. After all this, you'll find the radio will only hear the local 50 thousand watt flamethrowers in town. But with better audio response. You can try to boost the gain in the IF stage by bypassing the cathode resistor with a 0.1uF cap to ground. You can use a low voltage cap here, as there will only be a volt or two across it here.

Something to watch out for is 10KHz whistles caused by out of town station carriers if the bandwidth gets too wide.

The AM NRSC-1 standard is:
Contrary to popular belief, AM stations in the United States are not required to roll off audio above 5 kHz. For many years, no audio filtering was required, at all! In the early 1990s, however, a set of frequency response specs called NRSC-1 was adopted. Some type of standard was needed because, for years, many AM stations had been boosting the treble to compensate for the poor treble response of many tuners. This increase in treble had the side effect of causing more adjacent channel interference, which led radio manufacturers to further narrow the bandwidths of their tuners. The NRSC-1 spec specified a standard treble boost (pre-emphasis) curve. In addition to the treble boost, NRSC-1 required a sharp roloff above 10KHz. A related spec, NRSC-2 defined the amount of permissible emissions on nearby channels. The NRSC-2 requirements can be found in CFR 47, part 73.44. According to NRSC-2, any emissions 10.2KHz to 20KHz from the carrier frequency must be at least 25 dB below the carrier. Emissions 20KHz to 30KHz from the carrier must be at least 35 dB below the carrier, and so on.

If you have the typical modern digitally tuned AM/FM stereo receiver for your home audio system, you probably noticed the poor quality of the audio from the AM section of the tuner. No audio high frequencies at all (above about 4KHz). As stated above, AM stations broadcast audio up to 10KHz. Which makes their AM modulated signal have 20KHz bandwidth. The FCC assigns carrier frequencies further apart than this in your particular town. Out of town signals on adjacent channels are usually too weak to be heard on your local station. Most modern receivers use a ceramic filter of about 10KHz at most, yielding audio that tops out at 5KHz. What I did to a set that uses a Sanyo LA1851N AM/FM stereo chip is remove the AM ceramic filter and its IF and replace it with a set of 3 IF interstage transformers taken from an old narrow band FM pager transistor radio (GE model 4er35a12 if you happen to have such laying around). These will be aligned to the 450KHz IF. See diagram below. All you need are 3 interstage IF transformers, 2 100K resistors and a pair of 7pF caps. Here I cut out the section of the old pager circuit board that has these IF transformers and caps already wired and wired it to the stereo receiver board where the old ceramic filter and its IF was. This board has a buffer/amplifier transistor just after the last IF transformer, which I kept to make up for insertion loss of this new filter. I changed the old PNP germanium transistor with a silicon NPN 2N2222. Germanium transistors tend to be noisy. You could do a neater installation than this, but keep the signal wires as short as practical. I used teflon mini coax cable. You really need a scope, sweep and marker generators (see below in this web page) to do a proper alignment, where I got about ±1dB of passband ripple (a peak was right on the station carrier, which is a good thing to avoid detector distortion due to not enough carrier). Do be aware that selectivity will take a hit, but I was after local station hifi and not DX reception.

You could salvage the old transformer from the old filter (highlighted in yellow above), and use it as the first transformer of the new filter (highlighted in green above). Use the same section of the primary for the mixer's output and B+ when using it in the new filter. Just use the tap which used to connect to B+ and connect it to the mixer output, and the end that used to be connected to the mixer and connect it to B+. This will make the 3rd connection that was unconnected become a high impedance source to connect the 7pF cap to to feed the next transformer.

Stations sound much better now, I get most everything they transmit. This is a spectral plot of the detected audio from a local AM station. This was taken from an "S" sound from voice audio. Reasonably flat to 10KHz, the station's transmitter NRSC brickwall low pass filters to
9. 5KHz as you can see here.

A version of this mod applied to a Technics tuner using an AN7273 AM/FM chip. One issue I've run into is that I can make the filter look decent on the bench, using the sweep and marker generators and scope described further down this web page. But the response can get messed up installing it into the radio. So the use of buffer amp transistors, one on the input side and another on the output. The output buffer also includes some gain to make up the insertion loss of the filter. The AM mixer output runs around 0.18ma and I can use 39K and 150K resistors to provide a reasonable load for it. Note that this buffer transistor uses a voltage supply (14V) that is higher than that feeding the chip (approx 8V). This should provide enough head room for AM radio signals coming out of the mixer. As the filter has a peak at the AM radio station carrier about 2dB higher than the edges of the bandpass (this is a feature in the sense that this peak will help avoid detector distortion due to insufficient carrier) I added a small cap to bridge an audio level dropping resistor used in AM mode, but shorted by Q212 when the tuner is in FM mode. This will make up for the droop in the filter, to bring up the treble on AM radio stations. This may make for poor phase linearity or group delay in the audio output, but the ear may not much care.

Some chips don't like the input buffer, and it turns out it isn't really needed. As the AM mixer fed a tap on the first IF transformer of the old filter, and it feeds the same sort of tap on the first new transformer. I used a 100K resistor to inject the test sweep and marker generators into this circuit, this resistor provides high impedance isolation to avoid loading the filter. The scope probe goes on the output buffer collector, as before. I also bypassed the emitter to get some more gain.

The same mod, done in a Sony receiver that uses a LA1266 AM/FM chip. Its AM IF circuit is pretty much the same as the above chips.

A simulation of the 3 IFs:

And the group delay, which is symmetrical. 20usec differential group delay has no real impact on the demodulated audio anyway.

20KHz wide filter
Of course you could just replace the old ceramic filter with one of wider bandwidth. These tend to be hard to come by. Sometimes you can find usable ones in narrow-band FM receivers like old pagers and scanners.

If the set is digitally tuned, you must select a new filter to be the same IF frequency as the old one. Or else the set won't tune on channel correctly. They come in 450, 455 and 460Khz. If the new filter is off by 10KHz then the set will tune stations one 10KHz channel off, but otherwise sound fine. The tracking across the band will be slightly off. Worse yet is a filter 5Khz off. Then you can never tune the station right.

If the set is analog tuned (ie, slide rule or round dial with twist knob, with an old fashioned tuning cap) the worst that happens is that the dial calibration will be off slightly and tracking off slightly. Most dials aren't this accurate anyway.

A wideband ceramic filter in a Trivoli Model One's AM section

I picked up a Trivoli AM/FM set a couple weeks ago at a garage sale. Worked nice, and modified the AM with a wider IF ceramic filter I salvaged from an old analog (first generation) cell car phone. A MuRata CFL455AG2 about 18KHz wide, meant for narrow band FM work, but it doesn't know nor care that it's passing AM at 455KHz. I literally hacksawed the phone's circuit board to get the filter, as I didn't want to risk roasting the filter trying to unsolder it. This also provided me a little subboard to have the filter on. The 270uH coil on pin 3 of the TEA5710 chip (AM mixer output, an open collector) is intended to present around 1K impedance load on that mixer output, and thus approx 1K source impedance for this ceramic filter. Pin 2 is the AM IF amplifier, with an input impedance of 3K, thus I added a 1.5K resistor to make the load 1K on the output of this ceramic filter. I also added around a 330K resistor (actually I used a 2meg trimpot (under the board) connected reostat style to adjust this resistance so I could get bandwidth without killing the radio's sensitivity too much) to load the 2nd 455KHz IF LC circuit (pin 7) to broaden its bandwidth, so I don't lose the widebandedness of the ceramic filter. On pin 13 (audio out, approx 5K impedance) of the TEA5710 had a 0.01uF cap to ground, which rolls off the highs to much, IMHO. I changed it to a 2200pF cap, which brough up the highs on both AM and FM audio. Made doing the other nods here worthwhile.
I also replaced the crude loop antenna with a ferrite rod antenna. I removed many turns of wire to get it to resonate. And slid the coil form about the rod to fine tune this resonance, listening to a station around 570KHz to get a peak. Then touch up the antenna trimmer cap for the high end of the dial, around 1520KHz.

Here's some characteristics of the ceramic filter.

This filter came out of a first generation cell phone. Remember the cell phones of the 80's, the ones you could eavesdrop on with a scanner that could tune around 870MHz? Those used narrow band analog FM modulation, and used double conversion IF strips, the 2nd IF was 455KHz but the FM signal required a wideband filter, which I repurposed for wider than usual wideband AM reception. Many narrowband FM radios likely have such filters you could grab for this. Above right (Fig 2 from the NRSC-1-B AM spec, Sept 2012) shows the deemphasis receivers are expected to use, with corresponding preemphasis done at the AM radio station transmitter. I don't think many AM stations are actually doing this, as most stations don't sound shrill as my mods here would impose too little deemphasis to meet this NRSC-1-B spec...

A setup to look at passbands of filters

I'd like to see the passbands of various ceramic and other filters, and it turns out I have the equipment to make it happen. A frequency generator (VC2002 function generator), a sweep generator (Heath 1274 sweep function generator, and a scope (Tek TAS465). The setup:

I can adjust the amplitudes on both generators. I can see the shape of the bandpass (linear, not dBs though) and I can also see the amplitude of the marker frequency outside the passband of the filter on the scope. The scope is triggered from the sweep gen's "pulse" output (one pulse per sweep). The sweep is a frequency chirp, here from about 410 to 470KHz. The marker is always on. But the marker has to go thru the filter to get on the scope, and changing the marker frequency I can see when it drops to half amplitude and thus the 3dB points, high freq and low. The marker also beats with the sweep generator (which doesn't really show in the pictures) when the frequencies of the two happen to coincide.

The Device Under Test (DUT) is a ceramic filter at 455KHz, and has a little more than 20KHz bandwidth at 3dB. Which would be good for 10KHz of AM radio station audio, the max the FCC allows. Hifi AM. Okay... Yes, channel spacing is 10KHz, but not in the same town. Thus you can have decent sound on your local AM music stations.

Using an op-amp "rectifier" as an AM detector, then reconfigured as a peak detector for self biasing
Using a high speed op-amp, in this case a HA2525 (a high performance op-amp with a slew rate of 120V/uS and 10MHz unity gain), I built a "rectifier" circuit as an AM detector. Idea is that it should be more linear than an ordinary diode. This particular op-amp circuit outputs the negative going parts of the RF carrier and the amplitude modulation. Instead of a curved diode knee, this circuit would do a much sharper knee, thus better detection of weak signals and also strong signals (better audio fidelity on music). Regular detector diodes have a rounded knee around 0.7V, as shown below with the detected signal as a negative output, as the AGC circuit wants it. And what the op-amp rectifier would do.
. The RC circuits fed by the op-amp's output filters out the RF part of the rectified waveform to get audio, and AGC voltage.


I first built it as an op-amp rectifier circuit. I found I had to use a trimpot to feed some bias into the op-amp's + input to get decent detection. The trimpot setting was rather fussy, and could drift away from a good setting. I did some more work on it and made it a peak detector. I never much liked having to use trim pots, which usually means that the circuit isn't self bias correcting, and can easily get out of adjustment. Added a 3nF cap to the circuit output, and some resistance to the positive supply, here a 100K resistor. Also a 270K resistor to the + input of the op-amp, which also has a 910 ohm resistor to ground, used to minimise the error caused by the input bias current. Found that another resistance to the positive supply, a 270K resistor got the circuit in the sweet spot of detection (your resistance results will vary from mine). I think that these resistors are making up for the fact that my op-amp power supplies are not exactly the same absolute value voltages, and for the AGC line IF amp bias circuit. And it's not at all fussy, varying supply voltages doesn't mess it up. Substantially self biasing. I kept the 2nd diode, it acts like a catcher for the op-amp's output pin so it doesn't have to slew back so far when it comes time to rectify again .

Infecting a solid state stereo receiver with tube sound

Someone gave me a defective but free Sony SS receiver STR-D2020 that had a blown Sanyo driver module. Replaced that and the main SS outputs with tube outputs. Also modified the AM tuner section (changed the IF filter to one that passes 20KHz to deliver all 10KHz of audio AM stations broadcast, like above). But I also had to get some tube sound into all of this box, so I installed a cathode follower between the function switch chips (IC605 and IC606) and the audio line amp IC614 circuit that drives a DSP and then "surround sound" SS amp circuits and the new tube power amp. This is partially a case of "tube preamp feeding a SS power amp". It sounds good. Simulation says I get 2nd harmonic about 45dB down on a 2Vp-p audio signal, and almost 70dB down on the 3rd. Less higher up. Well, I like small doses of 2nd H...

I used a pair of 5906 subminiature sharp cutoff pentodes. Triode strapped, but is that significant in cathode follower service? Well, I had lots of 'em, also the 26. 5V heaters makes it easier to run off the SS power amp power supply (+65V and -65V). Also submini tubes are "cute", and small enough to shoehorn inside this receiver. Used the +65V supply with series resistor and with the two tube heaters in series. Okay, but how about B+? Well, I built a kind of voltage trippler circuit with a second bridge rectifier and some "AC coupling" caps. See circuit. I built this on a salvaged small switching power supply board, using its old AC line bridge rectifier and filter cap. This board made a handy way to mount this circuit. The coupling caps went where an inductor filter used to live.

The diagram below shows a simulated amount of power supply ripple (which is the amount I saw with a DVM via a cap after I built the power supply with a dummy load, but before I built the cathode follower circuit), and the tube's power supply rejection on the output. Which is pretty good.
More about the Wide band AM IF filter above.

And here is a solid state table radio (a Panasonic AM/FM model RE-6280) that I added a tube audio output to. The tube is a subminiature 5639 "video" output pentode, not a beam power pentode. Normally this tube isn't all that linear in pentode mode, so triode mode improves it. But oddly enough, inserting a 5K screen resistor into the triode wired mode makes it more linear yet. AFAICT, this is just special to this kind of tube. But this might be applicable to other oddball tubes otherwise not usable for audio.

The B+ is just directly rectified off the powerline, making this a hot chassis radio. But this radio chassis is easily isolated for safety.

The curves below show the 5639 in pentode mode, standard triode connection mode, and the modified triode connection, with the 5K resistor between the plate and screen, and the load taken off the plate and the 5K resistor node. Any load line centered at 150V and 30ma drawn on the pentode mode curves or the standard triode connection curves will have rather severe non-linearities. The modified triode mode has significantly more linearity here.

Cathode follower in a solid state tuner
This one is a Sansui T77 tuner. Added a cathode follower on the output audio jacks. And the power supply for the heaters and B+. For that tube warmth (2nd H about 45dB down according to simulations, and after building it, by ear). .Wonderful tube sound!
Here is a way to construct a powerline voltage reducer. For your vintage tube amplifiers and tuners. The transformer is configured to become an autoformer, the secondary in series with the primary. You need to check that the secondary winding phasing is correct, do that by measuring the voltage on the output (where the primary and secondary are connected), it should measure (if it's a 6V secondary) about 6 volts lower than the incoming powerline voltage. If it measures 6 volts higher, reverse the secondary winding connections, and measure it again. This autoformer conection will allow the transformer to run cooler than if it was the buck configuration. The autoformer configuration will also be able to deal with excessively high (like 127VAC in the USA) powerline voltages better as well. Though I'd check the rest of the outlets in the house, if all are equally high, call the power company. If some are high and others low, call an electrician, you may have a bad neutral connection.